The present invention relates generally to a pyrolytic boron nitride article and a method for producing the same, and more particularly to a pyrolytic boron nitride article adapted for use as a crucible for containing metals, such as epitaxy metals, in the evaporation process, as a boat for containing a material for semiconductor in the semiconductor growing process, as a jig, as a travelling wave tube support rod, as a material for forming a window passing microwave and infrared rays therethrough, and as a material for electric insulators.
Pyrolytic boron nitride is high grade boron nitride of high purity and used for wide industrial applications including crucibles for producing semiconductors and other special alloys.
Such pyrolytic boron nitride has been produced through the so-called chemical vapor deposition method, as disclosed by U.S. Pat. No. 3,152,006, wherein boron halide such as boron trichloride and ammonia are used as gaseous starting materials to deposite boron nitride at a temperature of from 1450.degree. C. to 2300.degree. C. and at a pressure of not more than 50 Torr, preferably not more than 1 Torr on the surface of an appropriate substrate or mold. Then, the deposited pyrolytic boron nitride is separated or released from the substrate to obtain an article made of self-standing pyrolytic boron nitride.
A self-standing article ofpyrolytic boron nitride, such as a crucible or boat, which must have high mechanical strength may be prepared by the chemical vapor deposition method carried out normally at a temperature of from 1850.degree. to 2100.degree. C. In the thus prepared pyrolytic boron nitride, crystallization of boron nitride forms a laminar structure wherein the C-planes {(001)plane} of the crystal lattices are oriented in the direction close to the parallel direction to the surface of the substrate or mold. Accordingly, the article made of such pyrolytic boron nitride has a high mechanical strength in the direction parallel to the surface of the substrate with improved anticorrosive property and thermal stability. The properties of an article made of pyrolytic boron nitride are in close relation with the microstructure thereof.
We have found that a pyrolytic boron nitride article prepared through the conventional method has a microstructure in which the non-uniform portion exists in high density or ratio, which induces deterioration of the original properties of the pyrolytic boron nitride material. FIGS. 1 and 2 are simplified diagram delineated from microscopic photographs wherein a cross section 10 cut along a plane transverse to the deposition direction, i.e. the direction perpendicular to the peripheral face of an article made of pyrolytic boron nitride and prepared by the conventional method is photographed through a microscope. Referring to FIGS. 1 and 2, the cut cross-section 10 has portions or areas each being composed of plural layers of deposited pyrolytic boron nitride arranged uniformly (shown in FIG. 1), and portions swollen to form generally hemispherical small lumps 1 whereat the laminated layers are locally disarranged as diagrammatically shown in FIG. 2. Examining such disarranged portions in detail, we have found that a generally spherical nucleus 2 having a diameter of about 5 to 50 .mu.m is present at the center of each hump 1. Such hemispherical small humps are referred to as nodules throughout the specification and claims. The product through the conventional method is ground by mechanical finishing to thus have a smooth surface. As a result, a number of small circular spots are found when observing the surface of a variety of commercially available pyrolytic boron nitride articles through a microscope, such spots being the traces of nodules.
Presence of such nodules or traces thereof causes serious disadvantages, such that they deteriorate the qualities of articles made of pyrolytic boron nitride to lower the mechanical properties, the thermal shock resistance and the lifetime when the article is used repeatedly. Details of such a nodule will be explained hereinbelow while referring to an exemplified case where a container made of pyrolytic boron nitride is used as a crucible for growing a single crystal compound semiconductor through the Liquid Encapsulated Czochralski method. In general, a pyrolytic boron nitride crucible is produced by depositing boron nitride over the surface of a graphite mold or substrate to form a pyrolytic boron nitride envelope (having a thickness in the order of 1 mm) and then separating the envelope from the mold. For this reason, the nuclei of nodules are present on the interior wall surface of the crucible, and tend to act as nuclei of crystal defects or sometimes contaminates a semi-conductor material, such as GaAs, during the step of melting the semiconductor together with a capsulant B.sub.2 O.sub.3. Moreover, at the step of removing the cooled B.sub.2 O.sub.3 from the crucible after the completion of crystal growth, some of the nodule nuclei could be plucked off from the interior surface of the crucible while adhering to the B.sub.2 O.sub.3 mass to leave pinholes or to result in de-lamination of the crucible wall, leading to serious reduction in usable lifetime of the crucible. As to the traces of nodules, such portions appear as the areas where the protruding laminar structures (hemispherical small humps or nodules) have been planed by grinding or other means so that a cross-section of the laminate exposed on the interior wall surface locally. Since the laminated pyrolytic boron nitride structure is structurally weak or poorly integrated along the interfaces of respective layers, de-lamination tends to occur at such areas when subjected to repeated rapid heating and quenching cycles. In addition, the molten material contained in the crucible often impregnates in-between the interfaces of the laminate layers to pose another cause for de-lamination. In a square rod having an extremely small cross section of 1 mm square and a length of several tens of centimeters, as in the case of a support rod for a travelling wave tube for transferring a microwave, the nodules each having a diameter of about 0.1 to 1 mm affect the mechanical strength of the rod adversely due to irregularities in microscopic structure. The conventional pyrolytic boron nitride product has another disadvantage that metal impurities contained in the material for the crucible tend to migrate into a compound semiconductor during the step of growing the latter in the former, whereby the electric properties of the resultant compound semiconductor suffer significantly.